1. Field of the Invention
The present invention relates to a uniform, continuous thin sheet of a TiAl intermetallic compound and a process for producing the same by using a twin-roll type continuous casting process.
2. Description of the Related Art
A TiAl intermetallic compound is a light-weight metallic material having a very high strength at elevated temperatures and an excellent resistance to corrosion. For example, a high temperature strength as high as 40 kg/mm.sup.2 at 800.degree. C. was reported in Metallurgical Transaction, vol. 6A (1975), p. 1991. Accordingly, due to these high-temperature characteristics thereof, a TiAl alloy is advantageous when used for gas turbine parts, automobile engine valves and pistons, disks and bearings for high temperature use, aircraft frames, and outer plates of ultrasonic passenger airplanes.
Nevertheless, although a TiAl alloy is light-weight and has a high resistance to heat and corrosion, and therefore is suitable for high temperature service such as turbine blades, it is difficult to form same by rolling or forging, due to a poor ductility thereof at room temperature.
Among the above-exemplified applications, a thin sheet of a TiAl intermetallic compound is particularly suitable for use as the outer plates of the airframe of an ultrasonic passenger aircraft, and accordingly, a process for producing a TiAl thin sheet having dimensions such as about 1 mm thick, 30 cm wide, and 30 cm long must be established.
Conventionally, a thin sheet of TiAl intermetallic compound is obtained by cutting an ingot, or by a sheath working as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-213361, but a sheet having a length such as described above has not yet been provided. The ingot cutting method has a problem of a poor yield of material and a difficulty of obtaining a uniform compositional distribution due to gravity segregation. Conventional hot plastic-working techniques include sheath working, hydrostatic extrusion, isothermal forging, and hot extrusion, but the current process conditions for these techniques lead to an essential difficulty in that the high strength at elevated temperatures (e.g., 200 MPa at 1050.degree. C.) and high strain-rate dependency of TiAl must be overcome. The above-mentioned J.U.P.P. No. 61-213361 discloses that sheath working requires an S-816 Co-based super alloy sheath and a rolling speed of 1.5 m/min at a rolling temperature of 1100.degree. C. Also, in the proceedings of the Japan Institute of Metals, Sept. 21, (1988), p. 24, it was reported that a strain rate of 10.sup.-2 to 10.sup.-3 sec.sup.-1 is required at temperatures of from 950.degree. C. to 1000.degree. C., and this makes it difficult to control the rolling temperature and leads to a low productivity rate.
Moreover, the above-mentioned conventional processes can provide only a small TiAl product having dimensions of, for example, 20 mm long, 10 mm wide, and 10 mm thick, and requires complicated processing steps, and accordingly, much labor and equipment.
Although Japanese Unexamined Patent Publication (Kokai) No. 62-256902 discloses a process for producing a TiAl intermetallic compound by using a fast cooling technique, such as a single roll process or a twin roll process, in which a molten metal is solidified by a fast cooling at a rate of 10.sup.4 .degree. C./sec or higher to obtain a solidified product in the form of a flake, it has not yet been reported that a continuous thin sheet of a TiAl intermetallic compound can be obtained.
Consequently, the conventional processes starting from a mass of cast material such as an ingot cannot practically produce a TiAl thin sheet having dimensions such as 1 mm thick, 30 cm wide, and 30 cm long, from the viewpoint not only of the product soundness but also of the productivity rate and the equipment required.